Tuesday, March 10, 2015

Humidity and the greenhouse: a tale of two variables

Moisture and temperature. That's what I somehow have to balance in my greenhouse.

I found out first-hand last year that humidity is important to control in the greenhouse. I am growing vegetables and flowers to transplant into the garden later this spring, so I need to find out what conditions are best for these plants.

Too much humidity
While there is no ideal humidity, you don’t want your plants to be too wet. Wet leaves can lead to fungi, such as Botrytis, or powdery mildew. When moisture builds up in the greenhouse, droplets form on the ceiling and drip down onto the leaves of plants. The splash can cause the fungus to spread to other plants, or it can hit the soil and splash up onto plants.

Botrytis blight on a peony leaf (http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/fungal-spots/botrytis-blight-of-peony.aspx)
Powdery mildew on a squash leaf (http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/fungal-spots/botrytis-blight-of-peony.aspx)
When the greenhouse heats up during a sunny early spring day, moisture evaporates and the warm air holds it. When the temperature falls at night, the air cools and the moisture condenses—back onto the leaves, soil, and greenhouse surfaces, causing drips from the ceiling. It is crucial to keep the plants dry from dusk to dawn.

Controlling humidity
Keeping the air moving during the day by way of vents and doors helps the suspended moisture leave the greenhouse. My greenhouse has one self-ventilating window in the ceiling, but one open window is not enough. You need to have moving air, so opening a door to create a cross breeze will help the moisture escape. However, if you want to keep the temperature up in your greenhouse (because inside it’s 80 degrees but outside it’s 45 degrees), opening the door is not a good option. That’s where a fan comes in. It will help move the air, especially if you point the fan toward the open vent from across the greenhouse. The other option is to keep your greenhouse warmer at night so that the temperature does not drop too much. This can get a bit expensive though.

My wall-mount oscillating fan
The self-opening vent in my greenhouse
Complicating matters is relative humidity, which differs depending on air temperature. The warmer the air, the more moisture it can hold. The higher the relative humidity, the higher the temperature at which condensation will occur.

Avoiding excess water
It is important that water does not puddle in the greenhouse during the day, which would then evaporate and cause unnecessary additional moisture. The amount of water plants get during the day needs to be just right and will depend on how hot it gets inside your greenhouse. You don’t want the top of the soil to dry out, for example, if you have just planted seeds. Also, it is better to water early in the day to allow plants to dry before evening.

Apparently it is best to alternate between venting and heating your greenhouse at least two to three times per hour in the morning and evening hours. For me, I have decided to only use the heater to prevent freezing temperatures, so I will likely see what kind of relative humidity drop I can achieve without dropping the overall temperature too much. In other words, I won’t be cranking the heater up just after using the fan. I will have the fan on a timer, turning it on low (the leaves should move slightly) and then off, on and then off, several times in the morning and the evening. I am thinking that I will only need to have the fan on for two or three minutes at a time to start, and then adjust as the weather varies.

The chart below indicates the temperature and relative humidity that is adequate to prevent fungal growth and/or disease:

A few questions remain:
  • My humidity gauge only gives me humidity and temperature, not relative humidity. How do I figure out the relative humidity?
  • If my temperature is 86 degrees and my relative humidity is 90%, is that good?

Today, I installed the foggers and misters in my greenhouse, as well as my fan. I preferred the foggers but used two misters and three foggers on each side. I tested their reach out today and ended up with way too much humidity in the greenhouse. I still have some finessing to do, but I am pleased with the layout. Details of the foggers/misters' spray radia, as well as pictures, will follow soon.

Seedling update: lots of basil up now! And one lonely cherry tomato. I have them in the highest, warmest spot in the greenhouse.

The basil is tiny but doing well.

One little tomato planted finally popped up (in back).

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